Step 1Stand a mango, stem end down, on a cutting board and use a serrated knife to cut from the top to the bottom, running the blade close to the pit. Turn the mango around and repeat on the opposite side. Use a spoon to scoop the flesh from the skin and transfer it to the jar of a blender. Cut away any usable flesh attached to the pit, peel it and add to the jar. Squeeze the pit over the jar to extract the juices from any flesh clinging to the pit. Repeat with the second mango.
Step 2Stir the sugar, lime juice, water and salt in the blender and purée until smooth. Taste the purée, and if it is fibrous, strain it through a sieve into a bowl.
Step 3To prepare the passion fruit, halve the fruit and scoop the pulp and seeds into a small bowl. Mash the pulp with a fork to liquefy, then stir the pulp and seeds into the mango mixture. (If using purée, stir directly into the mango mixture.) Cover and chill the mixture several hours, up to overnight.
Step 4Make the sorbet: Freeze the mixture in an ice cream maker according to the manufacturer's instructions. Pack into a chilled container, cover tightly and freeze, preferably for several hours, before serving. Remove from the freezer about 10 minutes before serving to make the scooping easier.